1. Field of the Invention
The present invention relates to a piezoelectric transformer for driving a discharge tube such as a cold-cathode tube or the like for use as the back light of a liquid crystal display or the like, and a discharge lamp device including the same.
2. Description of the Related Art
Cold-cathode tubes for use as a light source for the back light of a liquid crystal display have a high impedance of several M.OMEGA. when they are lit, and require a high voltage of at least 1 kV. The impedance while they are lit is decreased to about dozen k.OMEGA. to several hundred k.OMEGA., and they are driven at a several hundred volts. Conventionally, as a high voltage source for driving such a discharge tube, a piezoelectric transformer which can be easily reduced in size and for which the required power can be simply reduced has been used.
FIG. 11 shows an example of the piezoelectric transformer, which is a so-called Rosen type piezoelectric transformer, and makes use of a primary longitudinal vibration mode in the longitudinal direction. As regards the constitution of the piezoelectric transformer, an input voltage is applied across input electrodes 2a and 2b through terminals IN-GND on the input side. An output voltage boosted and generated at an output electrode, caused by the piezoelectric effect and the inverse piezoelectric effect, is output through terminals OUT-GND on the output side.
However, when a cold-cathode tube is connected directly to such a piezoelectric transformer, the tube current of the cold-cathode tube is oscillated in some cases as shown in FIG. 8, depending of the input voltage to the piezoelectric transformer and its drive frequency. In FIG. 8, time is plotted as abscissa, and the tube current as ordinate. The high frequency component of this periodic waveform is the drive frequency of the piezoelectric transformer. The current flows as if the tube current is amplitude-modulated at a lower frequency than the drive frequency.
Japanese Unexamined Patent Publication Nos. 10-125970, 10-14448, and 10-150230 describes piezoelectric transformers and cold-cathode tube lighting devices of which the purpose lies in that the above-described abnormal oscillation of a tube current is prevented.
In the piezoelectric transformers and the cold-cathode tube lighting devices using the piezoelectric transformers described in the above-mentioned official gazettes, basically, a capacitor element is inserted between a terminal on the output side of a piezoelectric transformer and a cold-cathode tube, so that the output impedance of the piezoelectric transformer is effectively enhanced, and the tube current is stabilized. Such a method of stabilizing a tube current by insertion of a capacitor element is very effective. However, it is necessary to provide an additional capacitor element. Thus, there arises the problem that the number of components is increased, and the manufacturing process is complicated.